He said it was a question of network effects and impact. Mozart died broke because of poor money management and the inability to reach a worldwide audience.
An elementary school teacher is only paid a small amount relative to most celebrities because their impact is small. They cannot reach more 10-30 students at a time.
A baseball player is paid millions because he (through cable TV) can impact/entertain many more anonymous people indirectly.
Is it fair? Probably not but there aren't incentive systems in place to pay good teachers according to their impact.
Clive James had a fascinating BBC documentary a while back, a multi-part series "Fame in the 20th Century". It's pre-Internet but I believe still very relevant.